Firestore: get document by ID from CollectionGroup

I want to get a snapshot of a Book by its document ID.

My structure looks like this:

           stores
          /      \
    Store1        Store2
      |               |
    books            books
    /   \           /  |  \
 Book1 Book2   Book3 Book4 Book5

(stores is a collection)

I tried with a collection group:

my_id = "XXXYYYZZZ" # I know that there is a book with this ID
firestore.Client.collection_group('books').where('id', "==", my_id)

But it looks for id field instead of document ID.

I tried also to read all Book documents and filter by ID in Python, but I have maaany books, so that's not an option.

books = firestore.Client.collection_group('books').get()
book = [b.id for b in books if b.id == my_id]

And I cannot define a specific path to the document, because I have multiple stores.

What can I do?